Storm Forecast
Valid: Wed 03 Dec 2008 06:00 to Thu 04 Dec 2008 06:00 UTC
Issued: Tue 02 Dec 2008 23:36
Forecaster: PUCIK

SYNOPSIS

The circulation over Europe is dominated by a significant trough, which stretches from Scandinavia to N Africa. Its position will change only slightly with slow eastward movement. Very strong upper and mid level flow is surrounding the trough with several vorticity maxima rotating around it. To the east, a large ridge will influence the weather over E Europe. At the surface, large anticyclones will cover NE Europe and SW Europe. Otherwise, shallow lows are occupying the region of the upper level trough. Two significant changes are expected to take place - the first one will be the arrival of a deep low, which will affect the British Isles by the end of forecast period and another one will be the rapid deepening of a low over the Ionian sea.

DISCUSSION

...Ionian Sea, southern Adriatic Sea, Greece, Balkan states...

A very strong southerly flow will settle over the region as the aforementioned through will approach from west. Airmass characterized by steep mid level lapse rates, exceeding 8°C/km will be advected from south over the warm SSTs, yielding moderately unstable conditions. GFS simulates more than 1000 J/kg in southern parts of the tongue of instability, which will copy the forward side of the surface low. To the north, instability should be less, but still, 500 J/kg of MLCAPE is anticipated.

Storms will be ongoing in the region from the beggining of the forecast period. Firstly, only isolated development is expected. Nevertheless, DLS over 25 m/s suggests organised convection and with SREH values over 300 J/kg, supercells might form with attendant large hail and severe wind gust threat. A tornado / waterspout threat will be the highest across the Adriatic coast, where the strongest LLS ( over 10 m/s) will exist.

A rapid deepening of the low is forecast by the evening hours as an mid-level impulse will reach the area. The central pressure is expected to be under 996 hPa by 06Z Thursday. This process will enhance the low level wind field and backing flow around the trough will provide very high SREH values - over 400 J/kg in the 0-3 km layer. Low level jet will develop on the front side with windspeeds above 30 m/s. Under the jet, LLS values will increase to more than 15 m/s. Overall kinematic setup will look impressive.

Strong mid-level forcing as well as surface convergence will result in widespread thunderstorm initiation, possibly over the Ionian Sea. Storms should quickly spread northwards, reaching the Adriatic Sea and Balkan states by the 06Z. Due to the very strong convergence, one or more linear MCS will probably form. Strong low level shear and helical flow should result in the development of bowing segments and LEWPs with some embedded rotating circulations.

The highest threat will be the severe wind gusts, which will be widespread in the Level 2 region and possibly reaching over 32 m/s. Also, with very high SREH 0-1 km ( over 300 J/kg) LLS over 15 m/s and very moist airmass, environment will be conducive to tornadoes. The highest threat should exist across the Adriatic coast and within Level 2 area, strong tornadoes might occur. Large hail should be limited, but is possible in case that supercells manage to form. High rain fall amounts are simulated with the storms, so flash flood threat will also exist. Severe thunderstorms and their attendant risks are expected to persist well into the another forecast period.

... British Isles...

A significant cold front might become a focus for severe weather activity as the low will start to affect the area. Our region of interest will be placed under the left - exit region of the jet-streak, suggesting dry intrusion at upper levels and a pronounced divergence. This factor might strenghten upward motion and produce strongly forced rain band. More than 800 J/kg of SREH is anticipated just ahead of the front so few vortices might develop within the rainband with a risk of few tornadoes. What is more, 30 m/s flow at 850 level suggest strong potential for severe gusts, which might be enhanced by the frontal rain band circulation.